Slow down. Slow way down.
I wrote a few weeks ago about how to deal with sweat during the summer. When it gets as hot as this, sweat ceases to be a concern. Avoiding heat exhaustion, heat injury, and heat stroke is the new priority.
Ideally, ride early when it is coolest. Evening is not a good time to ride. Road surfaces retain heat and radiate that heat all evening, countering the falling air temperatures. However, if you are a commuter, you may not have much choice in when you bike.
If you have to bike in the heat of the day, slow down. When I’m on my way home from work, I pedal only when I drop below 8 mph. That’s incredibly slow! I pass the time by calculating how many times faster it is than walking (almost 3 times). Or I pretend I’m sitting in a car. I never pedal hard when it’s hot. I use my lowest gears as necessary to climb easily. Fortunately, I don’t have any big hills to climb.
You don’t have to be a prisoner in your own air conditioned house and car to avoid heat injury. You do need to recognize the limitations of the human body. Your body will become conditioned to the heat, but it takes time. If you are already athletic, it will take days. If you are not as athletic, it may take 2 or 3 weeks. Of course, if you stay in the air conditioning, you won’t acclimatize. The key is to get out there in the heat, but minimize your exertion.
Drink before you are thirsty, and don’t forget your electrolytes. Personally, I love V8 which has 10 times the sodium and potassium of Gatorade, but pick the source that fits your philosophy and personality, whether it’s a soda, a health drink, or salty food. Be sure to include potassium as well as sodium.